In piston-type internal combustion engines, piston rings are used to prevent both losses of air and/or a mixture of air and fuel during the compression stroke of the pistons as well as preventing the products of combustion from entering the engine crankcase during the expansion stroke. During the compression and exhaust strokes of the pistons, lubricating oil is supplied to the cylinder walls of the engine. Such lubricating oil bathes the cylinders when the pistons are in any position within their stroke except at bottom dead-center. The rings of the piston scrape this oil into the crankcase during the downward intake and expansion strokes of the piston.
The rings of the piston are split so that there is a ring gap formed between adjacent opposite ends of each ring. The gap can be measured to determine if the appropriate ring size is being used and the split construction facilitates installation of the rings within associated ring grooves extending circumferentially about the piston. The gaps further enable radial expansion and contraction of the rings caused by temperature variation and also to permit the rings to radially expand against the cylinder walls to compensate for wear. As the piston rings and cylinder walls continue to wear, the rings expand further outwardly increasing the size of their respective gaps. Thus, the gaps serve an important role in proper functioning of the piston rings.
However, as well recognized by those skilled in this industry, the ring gaps are also detrimental to the performance of the engine. During the downward intake stroke of the piston, the gaps allow small quantities of the lubricating oil to enter the combustion chamber and likewise allow small quantities of the air or air/fuel mixture to enter the crankcase during the compression and expansion strokes of the pistons as well as some portions of the exhaust stroke. Oil in the combustion chamber is undesirable as it reduces performance, increases emissions, and increases oil consumption. The combustion gases entering the crankcase mixes with the crankcase oil causing it to degrade and produce noxious gases which eventually leave the crankcase as pollutants.
Heretofore, there have been several approaches taken to solve the problem of gas and oil blow-by past the piston rings in an effort to increase engine efficiency and decrease emissions and oil consumption. These efforts include increasing the ring pressure on the cylinder walls and adding additional grooves and rings to the piston. The first approach increases friction between the piston rings and cylinder walls and thus negatively affects fuel consumption and increases the rate of wear. The latter approach adds to the size, weight and complexity of the piston and still permits a certain amount of blow-by since each additional ring is disposed in its own groove and hence spaced from the other rings allowing gas and oil to continue passing through the ring gaps.
Multi-piece compression-type piston rings, have been proposed prior to this invention but are usually limited to the oil control ring of the piston. Such oil control rings include a pair of steel rails disposed in a single oil control ring groove of the piston and separated by a discreet spacer/expander element which maintains the rails spaced from one another and serves to force the rails with great radial tension against the cylinder walls when the piston is disposed within the cylinder. The rails are constructed much like the remaining compression rings of the piston but are generally much thinner and are made of steel with their rounded contact edge chrome plated. Because the rails are spaced from one another the oil and gases can pass through the gaps of the rails.
Many of the same problems mentioned above also effect piston-type air compressors, as well as other mechanisms which utilize pistons to raise the pressure of working fluids or to extract power from them (piston-based expanders).
Thus there is a need to provide a piston assembly having piston rings constructed and arranged so as to eliminate or substantially reduce the amount of oil and gas blow-by past the rings during operation of the engine, compressor or other machinery.